STRATOSPHERIC OBSERVATORY FOR INFRARED
ASTRONOMY (SOFIA)
E. E. Becklin1,2 and L. J. Moon1
Division of Astronomy and Astrophysics, University of California Los Angeles, 405 Hilgard Avenue, Los
Angeles, CA 90095-1562, USA
2Universities Space Research Association, NASA Ames Research Center, MS 144-2, Moffett Field, CA
94035-1000, USA
1
ABSTRACT
The joint U.S. and German SOFIA project to develop and operate a 2.5-meter infrared airborne telescope in a
Boeing 747-SP is now well into development. First science flights will begin in 2004 with 20% of the observing
time assigned to German investigators. The observatory is expected to operate for over 20 years. The sensitivity,
characteristics and science instrument complement are discussed. SOFIA will have instrumentation that will allow
astronomical surveys that were not possible on the KAO. A future SOFIA survey project related to astrochemistry
is discussed.
INTRODUCTION
The Stratospheric Observatory For Infrared Astronomy (SOFIA) is NASA's and DLR's premier observatory
for infrared and submillimeter astronomy into the next century. A Boeing 747-SP aircraft will carry a 2.5-meter
telescope designed to make sensitive infrared measurements of a wide range of astronomical objects. It will fly at
and above 12.5 km, where the telescope collects radiation in the wavelength range from 0.3 micrometers to 1.6
millimeters.
The telescope and 20% of operations will be supplied by Germany through contracts managed by DLR. A
team led by the Universities Space Research Association (USRA) has been selected by NASA to design and
develop the airborne platform; and to assemble, test, and operate the SOFIA observatory.
The development of the science instruments to be attached to the SOFIA telescope will be the responsibility
of the U.S. and German science communities. In the U.S., science instruments will be designed and built at
universities and national centers through a USRA peer review process.
SOFIA FIRST LIGHT INSTRUMENTS
A total of ten instruments have been selected and are now under development. The selection includes four
Facility Class (FI) Science Instruments (AIRES, HAWC, FORCAST, and FLITECAM), five Principal Investigator
Class (PI) Science Instruments, and one Special Purpose Principal Investigator Class (SI) Science Instrument. Two
of the PI Class instruments are being developed in Germany.
FIRST LIGHT EXPECTED IN 2004
SOFIA will see first light in 2004, and is planned to make more than 140 scientific flights per year of at least
8 hours duration. SOFIA is expected to operate for at least 20 years, primarily from Moffett Field in California, but
occasionally from other bases around the world, especially in the Southern Hemisphere. SOFIA’s typical line of
sight precipitable water vapor is 10m.
The SOFIA Science and Mission Operations Center (SSMOC), to be operated by USRA, will be located at
NASA Ames Research Center at Moffett Field, in the same hangar housing SOFIA. The SOFIA Program will
support approximately 50 investigation teams per year. SOFIA is primarily for use by the U.S. and German science
Name
AIRES
HAWC
FORCAST
FLITECAM
EXES
CASIMIR
SAFIRE
FIFI LS
GREAT
HIPO
Class
FI
FI
FI
FI
PI
PI
PI
PI
PI
SI
Table 1. SOFIA First Light Instruments
PI
Institute
Type of Instrument
E. Erickson
NASA-ARC
Echelle Spectrometer; 40 to 110 m, R~10,000
D.A. Harper
Univ. of Chicago
Far Infrared Bolometer Camera; 30-300 m
T. Herter
Cornell University
Mid IR Camera; 5-40 m
I. McLean
UCLA
Near IR Test Camera; 1-5 m
J. Lacy
Univ. of Texas
Echelon Spectrometer; 5-28m, R=1500 & 100,000
J. Zmuidzinas Caltech
Heterodyne Spectrometer; 250-600 m
H. Moseley
NASA-GSFC
Fabry-Perot Spectrograph 145-655 m, R=5-10,000
A. Poglitsch
MPE, Garching
Field Imaging Far IR Line Spectrometer; 40-350 m
R. Gueston
MPIfR
Heterodyne Spectrometer; 75-250 m
E. Dunham
Lowell Observatory Visible Occultation CCD; Photometer/Imager
communities, but it is also open to the international science community. Seventy-six percent of the observations
will be allocated by a USA peer-review process (managed by USRA), 19% will be allocated by a German peerreview process, and 5% will be allocated at the discretion of the USRA Observatory Director. The USRA
Investigator Program is currently being formulated. There will be a mix of Key Projects, Surveys, Service
Observations, and Small Investigations. The latter would be awarded annually; the first two investigation modes
could be multi-year projects. The program will also support the development of U.S. science instruments for
SOFIA. Some of these instruments will be facility instruments, which will be supported by the SSMOC for use by
general investigators. General investigators may also use PI science instruments, but support will be limited. A data
cycle system, DCS, for use with facility instruments will allow non-airborne astronomers the ability to easily
obtain SOFIA observations in a routine manner. It will also provide an archive of airborne data.
Fig. 1. The SOFIA primary mirror is now lightweighted
by 80% and is being polished at REOSC in France .
Fig. 2. The SOFIA SiC secondary mirror blank.
Work on SOFIA is presently being carried out in Germany and the U.S. The primary mirror (Figure 1) has
been ground, annealed and lightweighted by 80%. The secondary mirror (Figure 2) is made of SiC. Figures 3 and 4
show forgings of the inner and outer support cradle. The aircraft (Figure 5) has undergone pre-modification test
flights to verify and document its performance. The flight tests indicate that SOFIA's performance is significantly
better than its predecessor's, the Kuiper Airborne Observatory (KAO). The aircraft is presently being modified to
include the open-air cavity door (Figure 6).
Detailed tests and analyses have resulted in several design changes. The telescope will now be built with a
hydrostatic bearing rather than an air bearing. Changes in the aircraft door design have also led to improved
telescope and aircraft performance.
Fig. 3. Forging of the inner support cradle.
Fig. 4. Forging of the outer support cradle.
Fig. 5. The SOFIA 747-SP aircraft, originally dedicated as the ‘Clipper Lindbergh’.
Fig. 6. The first structural cut in the 747-SP aircraft on March 21, 2000.
SCIENCE POTENTIAL
SOFIA will provide a sensitive platform with high spatial resolution for both imaging and spectroscopy at
most infrared wavelengths, including those longer than the 200 m limit of the Infrared Space Observatory (ISO)
and Space Infrared Telescope Facility (SIRTF). Figure 7 and Figure 8 show a comparison to other missions.
Fig. 7. SOFIA photometric point source sensitivities are shown compared with KAO, IRAS, ISO, and SIRTF. On
source integration time is 1 hour.
Fig. 8. SOFIA spatial resolution compared with KAO, IRAS, ISO, and SIRTF.
With the parameters given under SOFIA Characteristics in Table 2, and the atmospheric transmission at flight
altitudes given in Traub & Stier (1976) (see Figure 9) we calculate that the background limited NEFD at 100 m in
a 30% band should be about 400 mJyHz-1/2 and at 450 m about 300 mJyHz-1/2. The corresponding 1- noise in an
hour integration should be 7 mJy at 100 m and 5 mJy at 450 m. The 1- line flux limit in 1 hour should be about
4 x 10-18 Wm-2 at a resolution of 103 at 100 m.
Table 2. SOFIA Characteristics
Nominal Operational Wavelength Range
0.3 to 1600 m
Primary Mirror Diameter
2.7 meters
System Clear Aperture Diameter
2.5 meters
Nominal System f-ratio
19.6
Primary Mirror f-ratio
1.28
Telescope’s Unvignetted Elevation Range
20-60 degrees
Unvignetted Field-of-view Diameter
8 arcmin
13 arcmin at optimum focus
Maximum Chop Throw on Sky
± 4 arcmin (unvignetted)
Diffraction-Limited Wavelengths
  15 m
Recovery Air and Optics Temp in Cavity
240K
Image Quality of Telescope Optics (at 0.6m)
1.5 arcsec on-axis (80% encircled energy)
Optical Configuration
Bent Cassegrain, chopping secondary and flat folding tertiary
Chopper Frequencies
1 to 20 Hz for 2-point square wave chop
Pointing Stability
< 1.”0 rms for first light
Pointing Accuracy
< 0.”5 if on-axis Focal Plane tracking
=3” if on-axis Fine-Field tracking
Total Emissivity of Telescope (goal)
15% at 10 m with dichroic tertiary
10% at 10 m with aluminized tertiary
Chopped Image Quality due to
=9.”1 for 80% encircled energy diameter
coma for ±4’ Chop Throw
=5.”8 for 50% encircled energy diameter
SURVEYS WITH SOFIA
SOFIA has the opportunity to obtain airborne surveys for infrared astronomy. Some surveys will be
encouraged by the operations center. Below we give an example of a spectral line survey.
The variety of instruments on SOFIA makes it possible to conduct molecular line surveys in wavelength
regions that are not accessible from the ground. From ground based observatories such surveys have been
performed by e.g. Blake et al. (1986) from 247-263 GHz, Schilke et al. (1997a) from 325-360 GHz and Schilke
(1997b) from 607-725 GHz.
A study is ongoing to determine the feasibility of a similar project for SOFIA, covering almost the entire
region from 30-330 m, which is inaccessible from ground-based observatories (Becklin, Davidson and Horn,
1999). The German PI-class heterodyne spectrometer GREAT, operating in several bands including 1.6-2.0 THz
(188-150m) with a resolution of ~106 will be especially useful for line survey studies, see e.g. Graf et al. (1998).
With GREAT, the beam size for the 150-188 m range with SOFIA is of similar size as with the CSO survey
conducted by Schilke et al. (1997a). The 325-360 GHz and 607-725 GHz surveys show typical lower limits for 3-
line intensities of about 0.3 K and 1 K respectively. Using Hot Electron Bolometers (HEBs) for the 1.6-2.0 THz
range of GREAT, one can estimate that the total integration time for this 400 GHz range should not exceed 20
hours in order to detect 3- lines at a level of 1.5 K. The SOFIA survey should be similar to the lower frequency
surveys since the lines should be stronger for higher frequencies. Advantage has been taken of the fact that the 1.62.0 THz band of GREAT will have two polarizations in each of the two beams, so that simultaneous observations
at two different frequencies are possible.
Fig. 9. Atmospheric Transmission vs. Wavelength in the Infrared. The figure shows computed atmospheric
transmission (Traub & Stier 1976) as a function of wavelength for aircraft (14 km) and mountaintop (4 km)
altitudes. The absorption is largely due to water vapor; figure is from Erickson (1995).
The wavelength ranges adjacent to the one from GREAT (150-188 m) can be covered at the longer
wavelengths by CASIMIR which is another PI-class heterodyne spectrometer with a resolution of R106 and at
shorter wavelengths by other GREAT bands and by the facility instrument AIRES, a grating spectrometer with
R104. With CASIMIR, the range from 188-330 m becomes accessible and with AIRES the range from 40-110
m.
The atmospheric transmission at SOFIA's operating altitudes gives access to almost all molecular rotational
transitions up to the lightest hydrids in the wavelength range from 30-330 m. The first light instruments can cover
this range with only small gaps from 30-40m and from 110-150 m. Those gaps however could be filled with
future instruments and/or upgrades of existing instruments.
REFERENCES
Becklin, E. E., Davidson, J. and Horn, J. M. M., Stratospheric Observatory for Infrared Astronomy (SOFIA), in
The Physics and Chemistry of the Interstellar Medium, ed. V. Ossenkoft, Zermatt Symp. pp. 444-449, 1999.
Erickson, E. F., SOFIA: The Next Generation Airborne Observatory, Space Sci. Reviews, 74, pp. 91-100, 1995.
Blake, G. A., Sutton, E. C., Masson, C. R., Phillips, T. G., ApJS, 60, 257, 1986.
Graf, U. U., Haas, S., Honingh C., Jacobs, K., Schieder, R., Stutzki, J., Array receiver Development at KOSMA for
the Submillimeter and Terahertz Spectral Range, Proc. of the SPIE, Vol. 3357, 159, 1998.
Schilke, P., Groessbeck, T.D., Blake G.A., Phillips, T. G., A Line Survey of Orion KL from 325 to 360 GHz, ApJS,
108, 301, 1997a.
Schilke, P., Phillips, T.G., Mehringer, D.M., First Wide Band Submillimeter Line Surveys, in The Far Infrared and
Submillimetre Universe, ed. A. Wilson, ESA Publication Division, The Netherlands, pp. 73-78, 1997b
Traub, W. A. and Stier, M., Theoretical Atmospheric Transmission in the Mid- and Far-Infrared at Four Altitudes,
Applied Optics, 15, 364, 1976.